Reception of frequency modulated waves



Sept. 17, 1946. M. ZIEGLER 5 2,407,353

RECEPTION OF FREQUENCY MODULATED WAVES Filed April 12, 1943 GEN ERATORWITH VARIABLE ATTENUATOR RECEKVER 30 2Z1? (7. Z. I8 5 VARIABLE 05c-REACTANCE ATTENUATOR V TUBE i RF lg f f; LF. AF. AMP MIXER MIXER AMEDlSCRlMlNATOR AMP MARC ZIEGLER.

INVENTOR.

,1 BY I ATTORNEY Patented Sept. 17, 1946 RECEPTION F FREQUENCY MODULATEDWAVES Marc Ziegler, Buenos Aires, Argentina, assignor to HartfordNational Bank, and Trust Company, Hartford, Conn., as trustee IApplication April 12, 194.3, Serial No. 482,785

, 1 The present invention is, concerned withYthe reception of frequencymodulated waves and more particularly with the maintenance in properlytuned condition of frequency modulatedre- I 4 Claims. ((1250-20) As amatter of fact, in communication practice, temperature variations aslarge as 60 C. maybe commonly expected, so that the same requirement ofintermediate frequency stability would ceivers, and with the preventionof the reception 5 lead tothe excessive requirement of 2X pe of unwantedsignals. degree C. when no thermostatic arrangement is In the practiceof reception of radio signals at usedL- l l r j fixed frequency, as forexample frequency modu- All these drawbacks are not presentfor'th'edelation-communicationreceivers;and also ampli signs of' the secondcategory inwhich theide tude modulation communication receivers, which10 sired frequency adjustment is obtained by 'auto have to stay tunedtothe frequency of the signal maticfrequency control commanded: by the i'nto be received under all circumstances, without coming signal.Forexample, reference maybe the necessity of any readjustment, twodifferent made t'omy copending United States'Patent aptypes of designshave been proposed and are beplications Serial Nof464380 filed November3; s usedi 1942 and Serial No. 478,705,'fi1ed MarchlOI, I943 In thefirst category, the correct frequency adwhere a method of automaticfrequency stabili justment is guaranteed only bythestability of zationis described together with'its application the selective circuits usedand therefore, in the to the reception of frequency modulated waves.places where the circuit frequency hasan impor The automatic frequencycontrol action can be tant influence on the correct adjustment of themade very efie'ctiva'and can correct with great receiver, the common L.C. circuit has been reaccuracy all possible deviations of the tuning ofplaced by a quartz crystal. Typical representathe'receiver. It has beenexplainedin. said first tives of frequency modulationreceivers usingthis application, that the deviations of a local oscilsystem usuallycontain two crystal o scillators, one later from a desired frequencycanbe made pracat higher frequency for the first frequency chang ticallyas small as "desired and the intermediate er stage and a second one at alower frequency frequency signal in afrequency modulation refor thesecond frequencychanger stage. i In that 'ceiver of that type willeasily be maintainedwith way, a constancy of the frequencyadjustment isin a. few hundred cycles equal to the correct frecbtained which is justequal to that of the crystal quency, even when the frequency of thelocal used. I d g l oscillator shifts away a frequenc'y'valuewhichisAreal drawback of this system, however, is that hundreds of times aslarge; as for instance 100'kc. the frequency of the crystals cannot bechanged In thatcaseno crystal is necessary, which is during 0 eration.Therefore, thefrequencyprean important advantage, but the drawbacks" of'cision required for these crystals, especially'the crystal controlreceivers of the first category'are crystal of higher frequency israther high in orreplaced in common receivers of the automatic der toinsure that a receiver of that type is really frequency control type byanother inconvenience. adjusted within narrow limits to'the fixed fre-The effective frequency control, of course'holds quency of thetransmission to be received. only as long as the desired'signal ispresent, and In frequency modulated. reception especially in the absenceof this signal the frequency adaccurate tuning is very important; first;because 0 justment which is nolonger controlled-may shift generally thecomparative immunity of frequency outside the range ofregulation, sothat the auto: modulation reception from impulsive interference maticfreq cy Control a t p -W h is dependent on that accurate tuningandsecsignal .comes in'.. In the absence of the desired ondly because aslight detuning of the receiver s'ignal,"the au o t q e y t I may maylead to unacceptable distortion owing to the '45 :moreo'ver'be madeoperative by anotherundesired fact that at-maximum frequency excursionthe signaLxthe receiver then adjusting itself to that instantaneousfrequency may run beyond the undesired signal. In both cases, thedesired conlinear part of the discriminator characteristic. "ditions canonly be restored bymanual readjust- The precision required in frequencymodulated ments, but these are of considerable inconvencommunicationpractice at 40 me. when it is deience in many practical instances andtherefore manded that the intermediate frequency be corunacceptable inthe specification of a communirect to within 0.5 kc. is not lessthan1.25 10 cation receiver. I which is a requirement making the crystalrather I now have found thatthis drawback of the p nsive. For .the samereason, the temperareceivers ofithe automatic frequencycontrol type turecoeflicient of the crystal must be very low. which are much superior inall other respects,

' oscillator providing It is obvious that the amplitude value .of thewaiting signal must be substantially smaller than the voltage developedat the place of-injection' by the transmission signal during thereception so as not to interfere with the desired signal during itsreception.

It is very commonly required in the specificationof a communicationreceived that the receiver shall be made insensitive to all signalsbelow a certain level. In or'derto fulfill this requirement, all kindsof complicated silencer or squelch arrangements have been designed forcutting down the amplification of some part of the receiver so long asthe incoming signal has not reached a certain minimum value.- Thisminimum valuefgenerally is adjustable in order to :make? the, receiveradaptable :to' different conditions of reception. The ,adyantage of sucha silencer or squelch arrangement is that, when the desired signal isnotpresent, noises from other; and less. strong signals, staticinterferences and internalspontaneous fluctuations are not reproducedand the receiver is completely quiet.

I have now found.- that a very convenient squelch action can be providedby the above mentioned waiting isignal. "As a matter of fact, thiswaiting signal can be made adjustable to any 1 valuelso' that anysquelch level may be obtained. Withmy invention. I therefore solve twoproblems at the same time: 'First,'-that of'improv-r.

ingthe security of operating conditions ofya frequencymodulated receiverof theiautomatic frei quencytcontrol type and" second that of providinga' very simple and very effective squelch device. 3 I have alsofoundsthat' it is not unavoidably necessary to obtain the waitingandsquelch signal from a special local oscillator, which would me'aritacomplication of the design, but that it can be derived from one of theexisting local oscillators of the receiver. 1

This and other details and features of my invention will bemadeolear inthe description of the figures, wherein Fig. 1 represents a blockdiagram of a frequency modulation receiver of the automatic frequencycontrol type with a separate crystal the waiting and squelch signalrequired. 1 g c i w "Fig; 2 represents schematicallya basic embodinientof a receiver wherein an" adjustable waitingandsquelchwsignal isobtained from a local quartz crystal oscillator .of the receiver.

In Figure l, S is a wellshielded signal generatonof frequencysubstantially equal to that of 'the'signa'l to be received, and of aconstancy better than that of the local oscillators used in the Ireceiver R, as for example a crystal oscillator.

This generator provides at the input of the receiver'a signal of smalleramplitude thanthe signal to be received but large enough to make thereceiver operate correctly and adjust itself plitude is smaller than theamplitude of the signalat the. place. ofjinjection. This desirableeffect is characteristic of frequency modulation receivers and -has beenexplained by H. Roder in the publication Electronics (May, 1937, vol.

'10, No.5, pages 22 to 25 inclusive, 60, 62 and 64) As pointedouttherein, a frequency modulation receiver is only responsive to thestronger of two signals. .l tirthermorewhen the amplitude of the weakerof the two ignals is less than one-half the amplitude of the strongersignal, the disturbing phase shift produced (and hence the disturbingequivalent frequency modulation produced) is never more than aboutone-two hundredths (1/200) of the legitimate phase shift-equivalent tothe permissible degreeof frequency mode ulation of the signal. Such. asmall disturbance cannot produce a significant interference effect. Inthe case of n amplitude modulated receiver,

the waiting signal will produce with the desired signal a beat frequencysignal which may be audible and therefore unacceptable. The onlysolution in that case would be to switch oif the waiting signalgenerated as soon as necessary.

The above mentioned property of the frequency modulated receiver can bemade use of for obtaining an adjustable silencer by providing the signaloscillator of Fig. 1., with a convenient adjustable attenuator. As amatter of fact if we inject a non-modulated signal of, for example, 50microvolts in the antenna, the receiver will respond only to signalslarger than said value and if no signals are present larger than 25'microvolts, the receiver will be perfectly quiet.

If used for that purpose, the injected signal may therefore be called asquelch signal, as it replaces the squelch arrangements known in theart. 7

,. holds for any level above the normal lower working limits of thereceiver which gen;-

erally, for frequency modulated communication receivers wherein asilencer needed, is very low, that is lower than 1 microvolt.

As a matter of fact,'the waiting and squelch signal does, not nee'dgtobe injected in the antenna of the receiver but at any point; before themixer. Thevalue of the signal injected then, of course, must beincreased adequately.

The disadvantage ofan arrangement suchas that ofv Fig. l is that itrequires in addition to a normal receiver a complete oscillator plusattenm ator whiohmakesithe feature a rather expensive one: .2 however,the effects described have been fully obtained withoutthe addition ofaseparate. oscillator. The waiting signal is pro videdloy one off thelocal oscillators normally present in the receiver, thefrequency of saidlocal oscillator being kept automatically at the correct value bystabilization from the other local oscillator which itself is quartz;stabilized.

This receiver'has therefore all the stabilityof a receiver with a, localcrystal oscillatorand the consequentguarantee of receiving thecommunication when it comes in, and all the automatic adjustability tothe incoming signal of a classical receiver of the automatic frequencycontrol type with consequent advantages with regard to selectivity andabsenceof distortion.

In the example given in Fig. 2 the signal frequency is is. After onestage of radio frequency amplification It the first mixer I2 is passedthrough, which operates with a quartz oscillator M of frequency toobtain a first intermediate frequency The second mixer 16 operates withan oscillator [8 at frequency w so as to get a second intermediatefrequency f1. The waiting and squelch signal is obtained as the nharmonic of the second local oscillator 18. This harmonic, as is wellknown, always exists in practice, in the anode circuit of the oscillatortube and may be obtained without further equipment. The intensity of theharmonic wave is adjustable by means of the attenuator 28. The secondintermediate frequency wave is amplified by the intermediate frequencyamplifier 22 and applied to the discriminator 24 which in turn controlsthe reactance tube 26 and energizes an audio frequency amplifier 28 anda loudspeaker 30 in known manner. The discriminator is adjusted so as togive a control voltage which is 'zero in value when'the frequencyapplied at its input is equal to the nominal intermediate frequency ii.The Ifrequency of the second localloscillator I8 is automaticallycontrolled by means of the reactance tube 26 on thegrid of which theoutput of the discriminator is applied.

It will readily be understood that the oscillator ill will be adjustedautomatically at such a frequency that the output of the discriminatorwill be as nearly possible zero, that is when the intermediate frequencyis as nearly as possible equal to the predetermined value ii for whichthe oscillator frequency will be practically and its n harmonic willhave substantially the frequency of the signal to be received. Thewaiting and squelch signal therefore works simply as described above forFig. 1.

For n any value may be chosen larger than 2 and smaller than a higherlimit determined by practical considerations. For lower values of n (3or 4) a good signal-to-image relation is obtained; for higher values ofn a better amplification of the first mixer stage may be expected as thefirst intermediate frequency is lower but the signal-to-image relationis poorer and it may become difficult to obtain enough squelch signalamplitude of the desired frequency. For a signal frequency of 40 mc. thevalue of n=4 seems to be convenient.

It must be observed that a. very careful shielding of the second localoscillator 18 is necessary in order to reduce the waiting signal pickedup directly by the antenna or other parts of the receiver down to thelowest limit of reception.

' This may present some difficulties, but-these difliculties can besolved conveniently by using the existing technique of standard signalgenerators and the like. i

Evidently, many other arrangements can be readily devised by thoseskilled in the art, to give the same results on the same principles. Thecircuit indicated is notto be considered as limitative since the choiceof the circuit maydepend on particular circumstances which do not effectin any way the fundamental nature of the 'invention. i

It must also be understoodthat'the present invention may be appliedeither to frequency modulated receivers of classical types or to thoseincorporating automatic frequency stabilization using frequencycounters, especially inductance type frequency counters, or those usingexcursion shrinkage and the like, as described in my 00-, pending UnitedStates patent-applications Serial Nos. 464,380, filed November 3, 1943,477,990 filed March 3, 1943, 478,705 filed March 10,1943 and 451,186,filed July 16, 1942.

I claim: a

1. A superheterodyne receiver for frequency modulated waves comprisingan input channel for receiving a signal wave, a first mixer stagecoupled to said channel to receive said signal wave therefrom, a wavesource coupled to said mixer stage for producing a first intermediatefrequency wave, a second mixer stage coupled to said first mixer stage,a controllable frequency oscillator generating a fundamental frequencywave coupled to the saidsecond mixer stage for producing an intermediatefrequency Wave havinga predetermined frequency value, an intermediatefrequencyamplifier coupled to the said second mixer stage afrequency-responsive detector coupled to the intermediate frequencyamplifier for producing a control voltage proportional to the averagedeviation of the said second intermediate frequency wave from the saidpredetermined value, means responsive to the said control voltage forvaryingthe frequency of the oscillator in a sense to reduce thedeviation of said second intermediate frequency wave from thepredetermined value to substantially zero, means to inject into saidfirst mixer a wave having a frequency'which is substantially equal tofrequency of the Wave applied from the input channel and which bears aharmonic relationship to the frequency of the said fundamental wave toproduce in the intermediate frequency amplifier an intermediatefrequency wave substantially the same frequency as the frequency of theintermediate frequency Wave and at the output of the detector 'acontrol'voltage substantially equal to the control voltage derived fromsaid second intremediate frequency waveresulting from the signal Wave,and means to vary the intensity of the said injected wave."

2. A superheterodyne receiver for frequency modulated Waves comprisingan input channel for a signal wave, a first mixer stage coupled to theinput channel to receive said signal wave there-,

from, a wave source coupled to said mixer stage for producing afrequency modulated interme-' diate frequency wave, a second mixer sta ecoupled to said first mixer stage, a controllable freplifier coupled tothe said second mixer stage; a i

7 frequency-responsive detector coupled to the in-'- terr'nediatefrequency amplifier for producing a control voltage proportional to the.average deviation of the said second intermediate frequency wave fromthe said predetermined value, means responsive to the said controlvoltage for varying theifrequency of the oscillator in a sense to reducethe average deviation of the said intermediate frequency wave from thepredetermined value to substantially zero, means to inject into saidfirst mixer a wave having a frequency which is substantially equal tothe frequency of the wave applied fromthe input channel and which bearsa harmonic relationship to the frequency of the said fundamental-wave toproduce i the intermediate frequency amplifier an intremediate frequencywave having a frequency substantially the same as the frequency of thesaid second frequency modulated intermediate frequency wave and at theoutput of the detector a control voltage substantially equal to thecontrol voltage derived from the said second frequency modulatedintermediate frequency ave, and means to adjust the intensity of thesaid injected wave to thereby adjust the intensity of the said thirdintermediate frequency wave to an intensity greater than a predeterminedvalue and less than coupled to said mixer for producing a frequencymodulated intermediate frequency Wave, asec- 0nd mixer stage coupled tosaid first mixer stage, a controllable frequency oscillator generating afundamental Wave and harmonics thereof coupled to the said second mixerstage for producing a frequency modulated intermediate frequency Wavehaving a predetermined intermediate frequency value; an intermediatefrequency amplifier coupled tothe said second mixer stage, afrequency-responsive detector coupled to-the intermediatefrequency'amplifier for producing a control voltage proportional to the averagedeviation 0f the said second intermediate frequency wave from the saidpredetermined value, means responsive to the said control voltageforvarying the fundamental and harmonic frequencies of the oscillator in asense to reduce the average deviaticn of the said second intermediatefrequency wave from the predetermined value to substantially zero, meansto inject a harmonic wave from the oscillator into the said first mixerto frequency wave and at the output of the detector a control voltagesubstantially equal to the control voltage derived from the said secondfrequency modulated intermediate frequency wave, and means to vary theintensity of the said injected Wave to thereby adjust then-intensity ofthe said third intermediate frequency wave to a value greater than apredetermined value and less than the intensity of the said secondfrequency modulated'intermediate frequency wave. r

4. A superheterodyne receiver for :frequency modulated waves comprisingan input channel for a frequency-modulated signal Wave, a first mixerstage coupled to the input channel to receive said signal wavetherefrom, a stable frequency oscillator coupled to the mixer stage, thefrequency of the oscillator being equal to where fs is the centralfrequency of the signal wave received, 11 is any number greater than 2and fl is a predetermined intermediate frequency, a second mixer st-agecoupled to said first mixer stage, a controllablefrequencyoscillator'generating a fundamental wave nominally equal tothe, frequency of the received signal Wave divided by n and harmonics ofsaid fundamental frequency coupled to said second mixer stage forproducing an intermediate frequency wave having a frequency nominallyequal to if, an intermediate frequency amplifier coupled to said secondmixer, a frequency-responsive detector coupled to the intermediatefrequency amplifier for producing a control voltage proportional to thedeviation of the intermediate frequency wave from the value if, meansresponsive to the said'control voltage for varying the fundamental andharmonic frequencies of the controllable oscillator in a sense to reducethe average deviation of the said second intermediate frequency'wavefrom the value fr to substantially zero, means to inject a harmonic ofsaid controllable oscillator substantially equal to the frequency isinto the first of said mixers to produce in the intermediate frequencyamplifier an intermediate frequency wave having a frequencysubstantially the same as the frequency of the "said second intermediatefrequency wave derived from the signal-wave and at the output of thedetector a control voltage substantially equal to the control voltagederived from'the said second intermediate frequency wave resulting fromthe signal wave, and means to vary the intensity of the V MARC ZIEGLER;

